Conventionally, butyl rubbers are produced by copolymerization of isobutylene and isoprene at cryogenic temperatures (.about.-90.degree. C.) using an aqueous aluminum trichloride initiator and chlorinated hydrocarbon solvents. Operating at cryogenic temperatures is expensive, it is difficult and expensive to remove all of the water and chlorine introduced with the initiator and the chlorinated hydrocarbon solvents represent a considerable environmental hazard. There is a need, therefore, for a less expensive and more environmentally friendly process for preparing iso-butylene based polymers and particularly butyl rubber.
As used herein the term iso-butylene based polymers includes polymers ranging from 100% polyisobutylene to polymers containing 50% polyisobutylene with up to 50% isoprene or other related comohomers such as chloroprene.
The titanium-based olefin polymerization initiator Cp*TiMe.sub.3 where Cp*=.eta..sup.5 -pentamethylcyclopentadienyl, activated with tris(pentafluorophenyl)boron, B(C.sub.6 F.sub.5).sub.3, in a toluene solvent has been investigated in detail by the present inventor and co-workers (J.Am. Chem. Soc., 115, 2543-2545 (1993) and has been used in earlier work to produce syndiotactic polystyrene (see commonly assigned previous U.S. patent application 08/108,968 filed 19 Aug. 1993, and Macromol. Chem. Rapid Commun. 13, 265-268 (1992)). In the latter paper, as well as in publications describing olefin polymerizations by similar compounds (J. Polymer Sci.: Part A 27, 1539-1557 (1989); Polymer Sci.: Part A 29, 1253-1263 (1991); Organometallics, 12, 3075 (1993)), it has been widely assumed that the mode of olefin polymerization in all cases involves the coordination insertion, or Ziegler-Natta mechanism. In this now classic mode of olefin polymerization, the olefin monomer binds to a catalyst site on the metal complex containing the growing polymer chain and the latter migrates to the bound monomer, thus generating a polymer chain lengthened by two carbon atoms.
Noting apparent inconsistencies between relative rates of polymerizations of various olefins by the Cp*TiMe.sub.3 -B(C.sub.6 F.sub.5).sub.3 initiator system and reactivity patterns anticipated for Ziegler-Natta polymerization, an hypothesis is now proposed which is totally without precedent, that the Cp*TiMe.sub.3 -B(C.sub.6 F.sub.5).sub.3 initiator system can behave as a carbocationic polymerization initiator with olefins susceptible to carbocationic polymerization, such as vinyl ethers and N-vinylcarbazole (see commonly assigned U.S. patent application 08/251,989 filed 1 Jun. 1994, and J. Am. Chem. Soc., 116, 6435-6436 (1994)). Concurrent with these discoveries, it has been found that the above-mentioned initiator/co-initiator system is effective in non-aqueous solvents and over the temperature range -20.degree. to -110.degree. C. for the polymerization of isobutylene, which is not generally known to be polymerized by organotransition metal compounds, and for the copolymerization of iso-butylene with various dienes such as isoprene (to produce butyl rubber). This invention provides a more economically favourable (higher possible reactor temperatures) and more environmentally benign (avoids use of chlorinated hydrocarbon solvent) process than that currently utilized (Encyclopedia of Polymer Science and Technology, vol. 8, 427-436, 546-547 (1987)) for the manufacture of butyl rubber, and provides a framework for the reconsideration of other potentially useful transition metal compounds for the formation of iso-butylene homo- and copolymers.